Because of their particular combination of properties such as transparency, dimensional stability under heat and dimensional stability, polycarbonates in general are used as materials for the injection molding, or injection stamping, of optical data carriers. In order to improve their processability, which generally takes place at temperatures in the range of from 300° C. to 400° C., additives such as mold release agents and stabilizers are generally added to the polycarbonate.
Aromatic polycarbonates based on bisphenol A are used especially for the production of optical data carriers. However, they can absorb up to 0.34 wt. % water, which can have an adverse effect on the dimensional stability of the data carriers. Improved dimensional stability is important, however, especially in the use of blue or blue-green lasers.
U.S. Pat. No. 6,391,418 describes substrates for data carrier media comprising a biphenyl derivative as additive for increasing the dimensional stability (low shrinkage).
In M. Ueda, Mitsubishi Engineering Plastics Corp., Technical Digest of Joint ISOM/ODS 2002 Waikoloa Hi., Aug. 7, 2002, page 33-35, the addition of small amounts of m-terphenyl to bisphenol A polycarbonate is described, which leads to a reduction in the water absorption. However, such biphenyl derivatives have the disadvantage that they are highly conjugated aromatic π-systems, which already absorb in the blue or blue-green spectral range. This is undesirable in the case of storage technologies that work in this wavelength range. Moreover, terphenyls are relatively rigid molecules, which have a negative effect on the mechanical properties in the mixture with polycarbonate.
Also relevant in the present context are the disclosures of: (i) branched polyformals and copolyformals, their preparation and use reported in German Patent Application DE10339357 (corresponding to US 2005049389 A1) and (ii) aromatic formals as additives for lowering the water uptake of polycarbonate, reported in German Patent Application 10338909(corresponding to WO2005021629) and (iii) polyformals and copolyformals with reduced water absorption, production and use thereof reported in German Patent Application 10333928 (corresponding to US2005020802) and (iv) polyformals as a coextrusion protective layer on polycarbonate reported in German Patent Application 10333927 (corresponding to US2005031855)
Accordingly, the possibilities described in the above mentioned prior art do not give satisfactory results in every respect. However, there is no indication in the prior art either that polyformals might be suitable as a blend partner for reducing water absorption.
The object was, therefore, to provide thermoplastic molding compositions which comprise polycarbonate having reduced water absorption and which accordingly exhibit better dimensional stability. In particular, the new disk formats with a higher storage capacity and optionally a lower disk thickness, such as, for example, Digital Versatile Disks (DVDs), require higher heat stability in comparison with CDs. The occurrence of material damage during processing to molded articles and the formation of a coating in the tool become more critical.
With the molding compositions according to the invention this object is, surprisingly, achieved by an improved quality of the data storage means and improved processability of the material by the injection molding, or injection stamping, process and a reduced water absorption and accordingly, ultimately, improved dimensional stability.